1. Field of the Invention
This invention relates to a method of making a high frequency focused transducer.
2. Description of the Related Art
Ultrasound imaging is now well established as an important medical diagnostic tool. This method relies on a device called a transducer to create a train of ultrasound pulses, in the frequency range from 2 to 10 MHz, that are radiated into the body. Echoes returned from the tissues being imaged are detected by the same transducer and transformed into electrical signals which can be displayed on a monitor. Image quality is primarily determined by the ability of the transducer to focus the ultrasound energy while the sensitivity of the imaging system is determined by the piezoelectric properties of the material that is used in the transducer. The ultrasound transducer therefore plays a critical role in determining the performance of an imaging system. Current state of the art single element transducers utilize spherically curved radiators which are machined from a ceramic material such as Lead Zirconate Titanate (PZT) or from a ceramic-polymer composite. PZT is usually the material of choice for clinical transducers because of its high efficiency and excellent electrical characteristics.
Recently, a number of new ultrasound imaging systems have been developed for visualization of the eye, skin, endoluminal structures and intravascular structures at frequencies greater than 20 MHz. Unfortunately, at higher frequencies ceramic transducers are difficult to fabricate. In particular, obtaining a high frequency focused transducer is difficult since the thickness of the transducer material, being less than about one hundred microns, is too small for accurate machining of the ceramic into a spherically shaped disk. Electronic focusing using an array of elements is also difficult due to the prohibitively small element to element spacing that is required. In light of these problems, many high frequency imaging systems employ a planar ultrasound transducer and either leave the beam unfocused or weakly focus the beam using a spherical reflector (mirror), both of which degrade the lateral resolution of the system.
A method of fabricating spherically shaped 50-100 MHz transducers using a piezoelectric polymer material (Poly(Vinylidene Fluoride)) is known. The flexibility of this polymer allows the fabrication of spherically focused high frequency ultrasound transducers by deforming the material about a spherical object. Unfortunately, higher losses, and a lower electromechanical coupling coefficient make this type of transducer approximately four to ten times less efficient than a ceramic transducer. Polymer transducers are also characterized by a low dielectric constant which make it difficult to efficiently couple electrical energy to and from the transducer when the area of the transducer is small. In spite of these disadvantages, their ease of fabrication in spherical geometries have made polymer transducers dominant in applications at frequencies above 40 MHz.
This invention seeks to overcome drawbacks of known prior art high frequency focused transducers.